Human prolactin has been identified as a separate entity from human growth hormone and a number of radioimmunoassays developed. One particular version has emerged as the standard radioimmunoassay based on reagents supplied by the National Pituitary Agency. This assay together with radioimmunoassays for estrogen, progesterone, relaxin, TSH and with urinary analysis for dehydroepiandrosterone, etiocholanolone and androsterone are being applied to different ethnic groups with varying incidences of breast cancer.